Flash memory is a commonly used type of non-volatile memory in widespread use as storage for consumer electronics and mass storage applications. Flash memory is pervasive in popular consumer products such as digital audio/video players, cell phones and digital cameras, for storing application data and/or media data. Flash memory can further be used as a dedicated storage device, such as a portable flash drive pluggable into a universal serial port (USB) of a personal computer, and a magnetic hard disk drive (HDD) replacement for example. It is well known that flash memory is non-volatile, meaning that it retains stored data in the absence of power, which provides a power savings advantage for the above mentioned consumer products. Flash memory is suited for such applications due to its relatively high density for a given area of its memory array.
A commonly used flash memory is NAND flash memory, in which groups of flash memory cells are serially connected with each other in a string between a bitline and a source line, and multiple strings receiving common row signals form a memory block. NAND flash memory offers a high bit density per unit area, especially when each cell stores multiple bits of data, thereby reducing the cost per bit for the NAND flash memory device. As should be well known to those skilled in the art, NAND flash memory arrays are typically formed on a plane of the substrate and thereby extend in a wordline and a bitline direction (ie. X and Y). These are referred to as a planar NAND flash memory array. Other factors for reducing the overall cost for a NAND flash memory device is to continue scaling down of device feature sizes using the most current lithography tools for reducing the physical cell size, and to increase the number of cells per string.
As the limits of semiconductor lithography are approached, new NAND flash memory fabrication methods have evolved to further reduce the cost per bit. One method is to increase the cell density by stacking the cells vertically on the semiconductor substrate. While this technique certainly increases cell density of a memory array, the reduced row pitch of each memory block relative to planar NAND flash memory blocks imposes new restrictions to certain circuits surrounding the memory array. These circuits are traditionally referred to as pitch-limited circuits as their layout is limited by the row pitch of the memory blocks, which are formed in the x-y plane of the substrate.